How do I copy one side of a box and paste it on the rest of the sides?
To "copy" the detailed side to all four sides, I use a array modifier, and an empty.


Start by adding an empty at the same location as your cube. Rotate the empty 90° along the Z axis.
Edit your cube to look like this, or without the top and bottom triangle.

Add a array modifier. Set the Count to 4, uncheck the Relative Offset and use the Empty as the Object Offset.







If you only want the detail on the opposite side


Add a loop cut down the middle (perpendicular to the X axis). 
Delete the half of the cube that you have not yet modeled.
Add a mirror modifier to the object.